1. Field of the Invention
The present invention relates to a rubber composition for coating a tire cord for a carcass or belt. More specifically it relates to a rubber composition for coating a tire cord having a reduced die swell and Mooney viscosity (ML), excellent workability (or processability), and satisfactory bonding property to a metal.
2. Description of the Prior Art
From the viewpoints of high speed durability and high speed steering property, steel cords are also used in radial tires. Where steel cords are used, a very high strain is likely to be concentrated on the rubber near the steel cords during tire running. Therefore, rubber for steel cord is required to have a high modulus of elasticity and an excellent bonding property to a metal. Also with radial tires or bias tires using organic fiber cords, rubber having a high modulus of elasticity is preferably used as the rubber for coating the cord, from the viewpoint of durability.
Various methods have been heretofore attempted for obtaining rubber having a high modulus of elasticity. A method comprising the addition of a large amount of carbon black involves an operational problem in that the workability of the rubber composition in a Banbury mixer is poor. In addition, the composition has an increased viscosity which generates a large amount of heat in an extruder, and thus, may cause scorching. In a method comprising the addition of a large amount of sulfur, blooming of the sulfur occurs and the vulcanizates derived from such a composition exhibit remarkably deteriorated physical properties, including flex resistance. A method comprising the addition of a thermosetting resin is disadvantageous in that since the thermosetting resin has a low compatibility with a diene rubber, which is usually used as the rubber for coating a cord, it is not uniformly dispersed throughout the composition, with the result that the resultant vulcanizate exhibits a poor crack resistance.
Also, conventional rubber compositions for coating a tire cord have a low green strength. Therefore, there is a desire for rubber compositions for tire cord coating having a high green strength from the viewpoint of workability.